Ballscrew actuators have been used for achieving relative movement between and positioning of members in many different environments, for example, for positioning of flight control surfaces of aircraft.
In certain applications, dual failure tolerant capability is required and one way of achieving this is by using two actuators, each having a ballscrew. There are a number of shortcomings in such a system to provide dual failure tolerant capability for moving and positioning mechanism of a space vehicle. The use of a pair of actuators can result in a failure in one actuator locking the entire drive train, thus preventing the other actuator from operation. To prevent this, a disconnect mechanism, such as a pyrotechnic device, pin puller or clutch is required or, in the case of a space vehicle, it may be necessary for a person to leave the vehicle and disconnect the disabled actuator. In addition, there must be a method of discriminating as to which of the actuators has failed, so that the correct one is disconnected.
The foregoing problems have been solved in various ways, but with considerable proliferation of parts and the solution has usually resulted in a number of elements which cannot be allowed to fail. Such elements constitute potential single failure points which can defeat the failure tolerance rationale of the actuator. These elements have to be designed to be noncredible failures, that is, they will not fail. Whether or not an element is a noncredible failure element can be the subject of controversy and normally a design goal is to minimize the elements requiring this designation.
The Weir U.S. Pat. No. 3,766,790 discloses a ballscrew linear actuator having redundant critical components. In the Weir patent structure, there are two drive mechanisms associated one with each of a pair of members to be moved relative to each other. Each drive mechanism has a ballnut rotatably mounted on a common ballscrew. In the structure of this patent, the ballscrew can be designed to be a noncredible failure, i.e., will not fail. However, there are a number of components which are subject to a credible failure, i.e., components that may fail. In the patent structure, a number of these components in addition to the ballscrew would need to be noncredible failure items to render the actuator single failure tolerant and an even greater number of these components would need to be noncredible failure items to render the mechanism dual failure tolerant.
The Grimm et al application Ser. No. 686,984, filed Dec. 27, 1984 and owned by the assignee of this application, discloses a jam tolerant redundant drive ballscrew actuator wherein a pair of primary drive units drive a ballnut. The two drive units operate through a gearbox to drive the ballnut. In the event of a failure of the primary drive units, there is a backup drive that can be brought into operation to cause operation of the actuator. A ballnut jam can result in halving the stroke of the actuator. There is also structure, such as a brake mechanism, which must be a noncredible failure item in order to make the actuator dual failure tolerant.
The ballscrew actuator disclosed in this application distinguishes over the prior art by providing full performance after any two failures of credible failure components while, in the prior art, the actuators are, for all practical purposes, single failure tolerant devices and with degraded performance resulting from some single failures of credible failure components.